This invention relates to doping of silicon single crystals, and relates more particularly to an improved float zone crystal growth apparatus and method for effecting controlled incorporation of doping materials, such as oxygen and/or n- or p-type dopants, into a silicon semiconductor crystal.
In the float zone (FZ) silicon crystal growth method, the silicon melt is not contained in a crucible. A single crystal is grown typically by moving a single-turn radio-frequency (rf) load coil upwardly in surrounding relation to and relative to a polycrystalline silicon feed rod. The coil transmits power directly into the silicon by induction heating at a frequency of about 2.4 MHz, creating a floating liquid pool of silicon that provides means to convert the polycrystalline silicon to monocrystalline silicon.
The silicon single crystals grown by this FZ method are doped by using gaseous sources during crystal growth. The gaseous dopants are diluted in an inert carrier gas such as argon and blown to the surface of the floating zone of the liquid silicon. This causes some of the gaseous dopants to be dissolved in the liquid in the float zone and as a result incorporated into the growing silicon crystal. The doping level is dependent upon the dopant concentration in the melt and its equilibrium segregation coefficient. Typically, phosphine (PH.sub.3).sub.2 or arsine (AsH.sub.3) is used for n-type doping, and diborane for p-type doping.
With gas doping, the FZ method must be carried out in an inert gas ambient using a slight overpressure relative to the atmospheric pressure. Ionization and plasma formation of the inert gas between the two leads of the rf load coil occur frequently during crystal growth. This causes perturbation of the steady state crystal growth and results in polycrystalline formation, which degrades the yield and throughput of the FZ method. Furthermore, the dopant gases (phosphine, arsine and diborane) are very poisonous, and require strict precautions for safe handling.
Oxygen in silicon crystals has some beneficial effects on the wafer during large and very large scale integration (LSI and VLSI) device processing. Precipitates of oxygen can act as gettering centers of impurities and thus improve the minority carrier lifetime in the wafer. On the other hand, if the oxygen concentration is above a certain level in the silicon crystal, the minority carrier lifetime may degrade in the wafer during processing. Therefore, it is desirable to control the oxygen level in the silicon to an optimum level tailored for the particular device applications. Although FZ silicon crystals are of much higher purity than crystals grown by the widely used Czochralski (CZ) method, the low level of oxygen obtained using the conventional FZ silicon method has heretofore prevented its application to LSI/VLSI device processing.
U.S. Pat. Nos. 3,858,549; 3,908,586 and 4,107,448 disclose typical apparatus for n- or p-type doping of semiconductor materials, such as silicon, in the conventional FZ crystal growth method.
There is a need to provide an improved FZ silicon crystal growth apparatus and method which (a) enables FZ silicon crystals to be doped from a solid source rather than with poisonous dopant gases, as taught by the prior art; (b) enables oxygen doping of FZ crystals in a controlled manner to achieve oxygen concentrations as much as about ten times higher than in an undoped crystal and close to the oxygen concentrations obtained in silicon crystals grown by the Czochralski (CZ) method, thereby providing high purity FZ silicon crystals which are suitable for LSI/VLSI applications; (c) facilitates doping with n-and/or p-type dopants by enabling them to be provided in desired controlled amounts as resistivity impurities in a fused silica rod that provides the desired oxygen concentration in the FZ crystal; and (d) enables increased yields and throughputs of FZ crystals.